Battery powered electric vehicle and electrical supply system

ABSTRACT

A charging system for a battery powered electric vehicle operates bidirectionally for charging the battery or for supplying power back to the utility grid at any selected power factor so that load leveling may be effected. A communications link between the utility and the charging system carries control signals and a control system associated with the charging system is responsive to the signals for controlling the charging rate and direction.

The present invention relates to electric vehicles and, moreparticularly, to battery powered vehicles having power transfer systems,either integral or separate, capable of transferring power betweenutility supplies and the vehicles.

The present invention further relates to battery charging systemsrequired for electric vehicles and so-called "electric-hybrid" vehiclesof the type having a main electric drive and an auxiliary internalcombustion engine (AICE) drive.

Environmental issues have heightened the interest in recent years inalternative means for providing personal and commercial transportation.Economic and regulatory issues have combined to promote the view thatelectric powered vehicles will, over the next ten years, appear insignificant numbers. It is possible that the number of electric orelectric-hybrid (EH) vehicles in key areas may be around 100,000 or moreby the year 2000.

Aside from the known modest performance levels of electric and hybridvehicles, a major issue is that of cost. Presently, EH vehicles withperformance levels acceptable to personal users are expected to sell atconsiderably greater prices than functionally comparable conventionalvehicles. Of particular importance here is the cost of the tractionbattery which is always much more expensive than a fuel tank. There areunlikely to be any compensating cost savings elsewhere within thevehicle, at least for the next few years. This means that a batterypowered electric vehicle will always have a considerable purchase costdisadvantage in comparison with a conventional vehicle. The day to dayrunning cost is dominated by the need to replace the battery every fewyears. Without this need, the running costs of an electric vehicle canbe very low.

Electric vehicles of this type require charging of their batteries fromtime to time, most commonly overnight. The charging process can takefrom less than 1 hour to perhaps 10 hours or more depending on theinitial state of charge and the charging rate possible. Electricutilities are keen to sponsor the use of large numbers of electricvehicles, primarily to increase their sales of electricity.

A major problem for utility electricity suppliers is the matching ofsupply to instantaneous demand. This involves a considerable amount ofplanning including estimates of TV audience patterns and weatherforecasting. The load can also vary significantly due to unforeseencircumstances.

The utility companies attempt to ensure continuity of supply whilstminimising total cost of generation. Several means are used to do this.Firstly, a utility will prefer to keep its most economical generatorsrunning where possible, though these typically take longer to bring upto speed and are expensive to use at very low loads. In parallel withthis it will keep some `spinning` reserve available. This reserveconsists of generators running at no or part load but synchronised withthe grid. These can generate additional electricity very rapidly. Thecost of maintaining such reserve is significant however. In addition tothis, utilities maintain a number of other generators in a state wherethey can be started and brought into use in perhaps 1 to 30 minutes. Ingeneral the smaller systems can be brought on-line faster but generateelectricity less economically than larger systems.

Other systems are in use, or being considered, including pumped storagesystems which use surplus, low-cost electricity to pump water from a onereservoir to a second reservoir located physically higher. Thisprocedure can then be reversed when additional power is needed. Batterystorage is also becoming more attractive, including the use of hightemperature batteries, though this requires additional rectification andinverter equipment to interface to the grid. The capital and runningcosts of this reserve capacity are high. Such techniques as aredescribed above are generally known as "load levelling" techniques.Recently there has been increased interest in providing means forcarrying out this function by giving utilities some control overconsumers' load pattern, usually by allowing utilities to remotely turnoff or reduce the power consumption of large power consuming loads. Thistechnique is known as "demand side load-management".

According to a first aspect of the present invention, a charging systemfor a battery powered electric vehicle has means for passing electricpower to or from its battery in either direction, to or from a utility(mains).

The charging system may be stationary, ie located at the normal point ofcharge, or else, preferably, may be integral with an on-board electrictraction drive system. Preferably, in this latter case at least, thebattery charging/supply system includes a number of semiconductorswitches.

In order that the direction and amount of power being passed can becontrolled locally or remotely by the electric utility company in such away as to match the electricity supply and demand, either on a grid or alocal basis, the vehicle charging system needs, preferably, to includeeither a timing means for arranging a connected charging system tosupply the mains grid at appropriate times, or else includes acommunication facility in order that the utility can send signals tovehicles to cause them to vary the power being taken during charging, orto reverse the power flow to supply the grid.

This system offers utility companies a potentially large rapid responsereserve capacity without having to have spinning reserve and at littleor no capital cost. This may make it attractive for them to subsidiseelectric vehicle batteries, thereby removing a barrier to the spread ofelectric vehicles and bringing forward the sales of electricity whichthey need for charging.

The invention thus also includes a mains/grid utility electric supplysystem having means for communicating with an electric vehicle, which isconnected with the grid/mains supply for charging, to cause the vehicleto transfer electric power from the vehicle battery to the supply. Minormodifications from the basic charging system as described above arerequired, for example, to allow communications to be received, andoptionally transmitted, to/from a local or remote command centercontrolled by the utility company, or others. Such a communication linkmay be based on signals superimposed on the utility supply or on aseparate channel such as a cable or fiber-optic link which mayadditionally be used for billing purposes.

The charging system described above inherently has the capability totransfer power bidirectionally, that is from the utility supply to thevehicle battery or vice versa as it is required both to drive thevehicle (battery to motor) and to charge the battery regeneratively(motor to battery). The system is based on semiconductor technology. Theselection of the direction and size of the power flow to/from the mainsgrid can be based on commands sent to the drive from the utilitycompany. It is thus possible for the utility company to command electricvehicles, which are connected for charging, to supply or take varyingamounts of power to/from the utility supply network. This can be donewith both single and multi-phase lines. The amount and direction can bechanged very rapidly. The utility company would have the ability tocommand connected electric vehicles singly, or in batches, to matchtotal demand and supply on their network. An advantage of the presentinvention is that this balance can be both network wide and local,thereby minimising transmission losses.

A further preferable feature of the present invention--as for theoperation of a delta charging strategy--is the use of an accuratestate-of-charge estimator used to control the maximum charge anddischarge states of the battery. In one possible implementation thisestimator can easily be integrated into the charging control system atlittle additional cost.

A further advantage of the present system is that it can be controlledin such a way as to operate at different power factors and/ordraw/supply controllable harmonic currents. These features are verybeneficial to correct for other loads on a network which cause a utilitycompany extra costs in the generation of reactive and/or harmonic power.

A further advantage of the present system is that it can be used tosupply power to key installations, or to the local home, commercial orindustrial location in the event of interruption to the main utilitysupply. In this event the charging system controllers could either becommanded to supply power using the communications link or couldautomatically detect the loss of the utility supply and initiategeneration in response to this. In this mode of operation the chargerwould generate its own internal frequency reference and supply an ACvoltage based on this. Where several electric vehicle chargers are usedin this way driving a common network, some means of synchronisation isrequired. In the system to which the present invention relates thesynchronisation would be carried out using the communication link or byusing a master reference generator in each local group. Some means ofpreventing generation when there is a fault on the incoming line isneeded, but this can be implemented by detecting voltage and current atthe interface to the utility supply and terminating supply if there is afault.

It is also necessary to consider the return of the utility supply, andthe changeover that must take place when this occurs. It is likely thata practical system must have the synchronisation signal supplied by theutility company. If this is done it will be possible to break andrestore the utility supply with little or no disturbance to consumers.This clearly needs a substantial infrastructure but is well within thebounds of existing technology for telephony and cable television and iswithin the scope of proposed integrated communication services todomestic, commercial and industrial properties.

The way in which this capability is used will clearly depend on thecircumstances of the particular utility, network and number of electricvehicles available. However, it is likely that the usage will be alongthe following lines: When buying an electric vehicle customers will havea dedicated charging interface installed at the point where the vehicleis to be charged, typically overnight. Similar sites will be availablefor charging at other locations, such as shopping centers, offices etc.At each of these interfaces will be a power connection and acommunications link. In exchange for some incentive, such as reducedpricing for the electricity used, the consumer will agree to the utilitycontrolling the direction of power flow, subject to criteria including aminimum state of charge of the vehicle's batteries. For example, it maybe appropriate to only use vehicles which have batteries at more than75%, say, of full capacity. The electric vehicle driver can overridethis function, for example if a very fast charge is required foroperational reasons, though this will lose the incentive for thatcharging cycle. Communication links to charging stations are alreadyunder discussion for remote billing based on an identification codestored in each electric vehicle. However, implementations can beenvisaged where simple operation is possible without a communicationslink in place, whereby a connected vehicle is programmed to take ordeliver set amounts of power at specific times. The programmed timescan, for example, be adjusted when the electricity consumption of thevehicle is recorded by the supplying authority. In a strategy like this,the vehicle logs the exact electricity consumption and the consumer isbilled regularly on an estimated basis which is confirmed by periodicreadings directly from the vehicle.

The utility supplier will be capable of estimating the potential poweravailable either from the knowledge that he already has about thetypical electric vehicle charging patterns, or by direct signals fromthe electric vehicle along the communication link for billing purposes.The utility can then signal single, or more likely groups, of electricvehicles to control their power take off or supply from/to the utilitynetwork. By scheduling and responding to changing demands the electricvehicles will act as a flexible load levelling system which eliminates,or reduces, the need to have spinning reserve or to build dedicated loadlevelling systems, as is current practice.

Although the cost differential between conventional and electricvehicles is, in a major part, due to the cost of the batteries needed,it is also due to the many support systems needed in an electricvehicle. The support systems problem is even more severe in an EHvehicle.

Such support systems include the supply for vehicle ancillaries and themeans for charging the traction motor battery or batteries.

For the first of these, means must be provided that deliver a powersupply for float charging of a battery which supports an on-board lowvoltage electrical system. Although an electric or EH vehicle hassubstantial levels of accessible electrical energy, this is invariablyat voltages incompatible with conventional vehicle components such asbulbs, switches, fuses, relays etc. The main electrical supply fortraction purposes is invariably above the Safe Extra Low Voltage (SELV)limit of 42 V, and may be higher than 300 V, so that reticulation oftraction voltages to specially rated vehicle ancillaries is dangerous aswell as inconvenient and costly.

In conventional vehicles, support for the low voltage supply is providedby an alternator or generator--rated at several hundred Watts--driven bythe internal combustion engine, together with regulating and controlequipment. This method is expensive, and although it can be applied toelectric and EH vehicles, it is typical with electric and EH vehicles touse a DC--DC converter to provide the low voltage supply directly fromthe main traction battery. Although the traction battery isconventionally made up of cell blocks, it is not possible to utilise oneof these individually. Aside from the safety aspect, using one tractionblock, or a number of traction blocks, to support the low voltage supplycauses unbalanced discharge and eventual battery damage. The DC--DCconverter is also expensive, costing $200 to $400 or more, and addsweight.

Traction batteries in EH vehicles are recharged from the normal AC mainscurrent using a charging system, but such conventional charging systemsare very bulky, weighing many kilograms, and are (in the case ofindustrial electric vehicles) mounted off the vehicle, at the homelocation. For personal electric vehicles, it is common practice to carrythe charger with the vehicle, so that charging can be carried out atdestinations where there is a convenient AC supply. Such on-boardchargers normally use high frequency switching technology to provide acharging unit of acceptable weight, in the tens of kilograms.

A second problem arises when energy flow rates are considered. A typicalvehicle traction battery will be rated at (say) 20 kW-hr. A single-phaseAC line can deliver energy at the rate of 2 to 3 kW, or more in somecircumstances, so that full recharging is possible in around ten hoursif perfect efficiency at unity power factor is achieved. In practice,the charge efficiency of the batteries may be as low as 80%, the energyefficiency of the charger may be 85%, and the charger may draw linecurrents with a power factor of 0.47 or lower. Consequently, charging inadequate time can be a problem, and energy costs can become unattractivewith simple systems. Furthermore, the injection of harmonic currents tothe AC network caused by simple charging systems may also raise bothtechnical and legislative difficulties, particularly if very largenumbers of chargers come into use.

Solutions to this second problem include provision of higher-ratedsingle-phase lines, and utilisation of multiphase supplies. Althoughacceptable, availability of these supplies is not universal. Alternativetechnical approaches include using special charging systems that drawline currents at near-unity power factor, and the adoption of "delta" orcharge/discharge methods that permit batteries to accept energy athigher rates, where an appropriate supply is available. Such approachesare expensive--with a high component cost--and add weight to thevehicle.

The traction system itself utilises well-known pulse-width modulation(PWM) inverter methods to synthesize a closely-controlled AC supply,from the DC traction battery. The controlled AC supply is used to driveconventional induction, permanent-magnet synchronous or other motorsunder variable speed and torque regimes to meet the demands of thevehicle user. Such systems are applicable from small ratings, up toratings of many hundreds of kilowatts.

Inverters of this type are not new, and have been used in small numbersfor industrial drive systems from the mid nineteen sixties. Early workin the field is described in Schonung, A. and Stemmler, H.; "StaticFrequency Changers with `Subharmonic` Control in Conjunction withReversible Variable-Speed AC Drives", Brown Boveri Review, Vol 51, p.555(1964).

The major advantage of such systems is the brushless nature of themotor, which has markedly lower cost--and higher environmentaltolerance--than the brushed DC motors normally used for controllabledrives.

Since 1985, drive systems of the AC type have come into more generaluse. An industrial inverter drive normally operates by first convertingthe normal three phase or single phase line supply to an intermediate DCvoltage, prior to "inverting" the DC back to AC with the desiredparameters for driving the target motor. This intermediate rectificationprocess complicates the drive, adding to cost, and has played a part inslowing the spread of AC inverter drives for industrial applications. AnAC drive is, however, well suited to vehicle traction applications wherethe primary energy source is DC batteries. Vehicle applications of ACsystems are still in the minority compared to conventional DC brushedtraction systems, primarily due to the sophistication of the controlsystems necessary to achieve satisfactory operation with the ACsystem--and the costs of such systems when conventional methods areused.

In the system to which the present invention relates, the inverter isbased on insulated-gate bipolar transistors, operating under the controlof a microcomputer. Many other device technologies are also applicable,and other control methods aside from PWM can also be used. An example isthe Load Commutated Inversion method, relying on natural commutation ofthe inverter devices, which is particularly applicable with permanentmagnet machines.

The inverter has an energy efficiency of approximately 96% at full load,so that when 50 kW are being delivered to the traction motor or motors,2 kW is dissipated in the inverter. At lower power levels, the lossesare not as substantial. However, it is rare for the losses to be lowerthan 1 kW.

Most inverter drives inherently offer regenerative operation.Consequently, energy is recovered to the batteries when "engine braking"occurs, and this effect is exploited in the majority of personalelectric and EH vehicle drives.

It is possible also to utilise the bidirectional energy flowcapabilities of the system which is the subject of this invention infurther ways.

According to a further aspect of the present invention, a traction drivesystem, for an electrically driven vehicle, includes an AC tractionmotor; a pulse width modulated (PWM) converter controllable to convert aDC electrical signal, fed from its battery to a first port, into an ACdrive signal for the motor fed out from a second port; an AC input port;and switch means connected to the second converter port, for switchingthe second port between the traction motor and the AC input port,whereby the AC input port, on connection to a suitable AC source can beconnected to the converter to charge the battery.

Preferably, the drive system comprises a traction battery of lead-acidor NaS type (although other types may be equally usable), plural DC linkcapacitors for sourcing and sinking high frequency current pulses thatresult from the operation of the power converter, a PWM converterconsisting of six unidirectional self-commutating semiconductor switcheswith anti-parallel diodes (insulated gate bipolar transistors arepreferable, but any suitably rated self-commutating switch can beutilised) the switches being arranged in a standard full-wave six-pulsebridge configuration, a microprocessor based controller one function ofwhich is to generate six drive pulses to provide gating signals to thestatic switches, an electromechanical changeover switch consisting oftwo mechanically interlocked contactors which allow connection of thePWM converter to either the AC traction motor or the AC utility supplyinlet port. The position of the changeover switch is controlled by themicroprocessor, but an additional interlock feature will be includedwhich inhibits changeover occurring whilst the vehicle or the tractionmotor is in motion. The traction motor may be of the synchronous,asynchronous or switched reluctance type.

The invention also includes apparatus for providing an auxiliary DCsupply for vehicle equipment, from a symetrical multi-phase tractionsupply which employs a PWM controlled converter, the apparatuscomprising means for offsetting a neutral point of the multi-phasesupply by offsetting the PWM sequences for each inverter output pole.

One example of a grid system and a vehicle employing a traction driveand charging system in accordance with the present invention will now bedescribed with reference to the accompanying drawings, in which:

FIGS. 1 to 6 are diagrams illustrating the principles of the invention;

FIG. 7 is a basic circuit diagram illustrating an inverter drive systemembodying the principles of the invention;

FIG. 8 illustrates the vectorial relationship of the converter polevoltages, leading to a neutral point offset, according to the secondaspect of the invention;

FIG. 9 is a flowchart which illustrates a charging regime which may beemployed in a vehicle according to the invention;

FIG. 10 illustrates a line current waveshape associated with a chargingregime of the type described in relation to FIG. 9;

FIGS. 11A and B illustrate devices which may be used for handling lowbattery voltage conditions when recharging;

FIG. 12 illustrates the alternative use of an additional converter whichmay be used for handling low battery voltage conditions whenre-charging;

FIG. 13 is a diagrammatic illustration of a grid/consumer systemaccording to the invention;

FIG. 14 is a diagrammatic illustration of a vehicle charging/supplysystem; and,

FIGS. 15A and B are diagrams illustrating details of filters which mayemployed in the circuit of FIG. 7.

The basic circuit elements for a three phase inverter drive are shown inFIG. 1. The basic inverter circuit comprises a pulse width modulatedconverter consisting of six self-commutating semiconductor switches T₁-T₆ with anti-parallel diodes D₁ -D₆ arranged in a standard full-wavebridge configuration. The diodes D₁ -D₆ are required where the switchesT₁ -T₆ are capable of unidirectional current conduction only. Amicroprocessor based controller μmC has the function of generating thesix drive pulses necessary to provide gating signals to the switches T₁-T₆. The PWM converter thus converts a DC supply U_(D) (usually abattery source) and supplies the resulting AC signals to an AC tractionmotor M. The traction motor M may be of the synchronous, asynchronous orswitched reluctance type.

Substantially identical circuitry is sometimes used to give"regenerative rectifier" operation, interchanging power between an ACsource and a DC source. A modification of this principle is utilised inthe proposed vehicle drive, so that the traction inverter components areutilised--when the vehicle is stationary--as a regenerative rectifier.

The operating principles are best illustrated in the single phasecontext. As illustrated diagrammatically by FIG. 2, an AC utility supply(or "mains") U is separated from an AC/DC converter C by a lineimpedance Z. The converter in turn is connected to a DC supply V_(LK).Where the converter is sinusoidally PWM controlled, an AC source E isgenerated at its terminals derived from the DC supply. The system canthen be represented as two independent AC sources separated by a lineimpedance Z (FIG. 3). The impedance Z is normally predominantly orwholly inductive in character.

Taking the mains or utility voltage as a reference, the vectorialrepresentation of the system is as in FIG. 4, and pwm control of theconverter C enables variation of the phase and magnitude of theconverter-generated AC voltage E with respect to the utility voltage U,as shown in FIG. 5. This control enables forcing of the current vectorin the AC network, and it can be shown that the current vector may beforced to lead/lag or be in phase with the utility voltage. Unity powerfactor operation may be achieved in both rectification and regeneration.In the simplified case where the line impedance is wholly reactive, thelocus of the voltage vector is in quadrature with the utility referencevoltage U. This is illustrated by FIG. 6.

FIG. 7 illustrates the basic circuit diagram of one possible systemaccording to the present invention. A battery 1 supplies a DC voltage toa PWM converter 2 which is in the form of six self-commutatingsemiconductor switches Q₁ -Q₆ with anti-parallel diodes D₁ -D₆ arrangedin a standard full-wave bridge configuration. A microprocessor basedcontroller 3 controls a drive board 8 which generates the required drivepulses necessary to provide gating signals to the switches Q₁ -Q₆. ThePWM converter thus converts the DC supply and supplies the resulting ACsignals to the contacts on one side 4a of a solenoid operatedmulti-contactor switch 4. To the other side 4b of the switch 4 areconnected the AC traction motor 5 (of the synchronous, asynchronous orswitched reluctance type) and, through appropriate impedances 6, theterminals 7a,b,c of an input port 7, to which a mains supply may beconnected when the vehicle is stationary.

In normal operation, the switch 4 connects the converter 2 to the motor5, the microcontroller 3, through the drive board 8, controlling theconverter switches Q₁ -Q₆ as required to drive the motor. Themicrocontroller 3 also controls the solenoid 4c of the switch 4 throughan amplifier 9 and is arranged to actuate the switch when called upon todo so by the operator so that a mains supply can be connected to theconverter 2 through the input port 7 to charge the battery 1. Thecontroller includes an interlock function such that the switch cannot beactuated when the vehicle is moving or the traction motor is operating.To this end a sensor 10 monitors the motor's speed and connects to themicrocontroller to provide speed signals. In order to enable charging ofthe battery, the microcontroller actuates the converter switches Q₁ -Q₆as required to convert the AC mains signals to a DC voltage across theterminals of the battery, along the same lines as a "regenerativerectifier", as described in Destobbeleer, E. and Seguier, G.; "Use ofPulse Width Modulated Techniques to Improve the Performances ofRectifiers", Proc. 2nd European Conf. Power Electronics, Vol. 1,Grenoble, 22-24 September 1987, and briefly as outlined below.

This system thus has the advantage, for the addition of a small numberof inexpensive components to the drive, of providing sophisticatedcharging compatible with many input sources--without a separate chargingunit as such. Delta charging is also possible with this system, so thatthe batteries can be rapidly charged, and indeed discharged economicallyinto the AC source if necessary to prevent memory effects.

A further advantage of the present system is that it minimizes thecomplexity of the physical infrastructure needed off the vehicle forcharging purposes, thereby reducing the cost of providing facilities forcharging electric and EH vehicles, particularly where high-rate chargingservices are needed. This will encourage the spread of power pointssuitable for charging, and hence increase the attractiveness of electricand/or EH vehicles to users.

While in most circumstances a utility mains supply at a frequency of 60Hz, or a similar low frequency, will be used as the charging source, itis a further preferable feature of the invention that a wide range ofsource frequencies can be accepted, including DC and/or frequencies ofseveral kilohertz. The upper frequency limit is principally determinedby economic factors such as the cost of the switching devices Q₁ -Q₆shown in FIG. 7. However, with normal designs a source frequency in theregion of three kilohertz can be accommodated, and ultrasonicfrequencies in the region of thirty kilohertz are possible if switchingdevices optimized for speed are used in the system illustrated by FIG.7. This aspect of the invention is important in situations wherein forlegislative or other reasons a non-galvanic (eg inductive) couplingbetween the charging source is required. Where inductive coupling (say)is required, power is reticulated to the vehicles to be charged at afrequency of several kilohertz (and at the appropriate voltage level).In multivehicle charging stations this reticulation would advantageouslybe from a centrally mounted large converter, which transforms power fromthe normal low frequency source, to the frequency required for charging.To accommodate an inductive interface, the line impedance 6 in FIG. 7 isreplaced by a two part transformer. The secondary winding of thetransformer is carried as part of the vehicle and connected to theswitch 4 shown in FIG. 7, while the mating primary of the transformerforms the utility connector interface 13 and is held as part of thestationary charging infrastructure. The high frequency of reticulationis important in reducing the physical size and weight of the thecoupling transformer required.

A further important aspect of the invention is that "delta" chargingregimes are easily implemented, wherein the batteries can be dischargedinto the connected utility network without dissipation of energy, aswell as recharged. This procedure is important where very rapid charging(for example, at the 1/2 or 1/4 hour rates) is needed, and involvesusing brief or sustained periods of discharge current to preventelectrode polarization effects. Electrode polarization occurs duringcharging through the finite time needed for ion migration from theelectrode surfaces. Periods of discharge reduce these effects markedly,and permit the total time required to reach full charge to besignificantly reduced, with lowered levels of battery self heatingduring charging. Discharging is also used for conditioning of new andpartially aged batteries (for the avoidance, for example, of so-called"memory" effects). The operation of a delta charging regime is describedwith reference to the flowchart of FIG. 9.

According to the delta charging regime exemplified by FIG. 9, energy isdelivered from the charging source to the batteries for a brief period(ranging from less than one second to several seconds or longer).Following this delivery period, energy is then withdrawn from thebatteries for a period shorter than the delivery period, so that thereis a nett energy flow from the charging source to the batteries. Restingperiods, where energy is neither delivered or extracted, are alsopossible. Such strategies are well known for rapid charging ofbatteries. In the improved regime which is used as part of thisinvention, information regarding the battery state of charge is used todefine the parameters (periods, energy delivery and extraction levelsetc) of the delta charging regime. In this fashion, specific incrementsof charge can be added to, or extracted from, the batteries, with theincrements being selected to be the most appropriate with regard to theprevailing battery state of charge. Instantaneous measurements of thebattery terminal voltage, and measurements of other parameters, are alsoused to control the charging processes. For example, if during acharging period (where energy is being added to the batteries) theterminal voltage is observed to rise above a predefined level, chargingis terminated, and a resting or discharging period is commenced.

The discharging abilities of the system described here can also be usedin a variety of ways to correct line current conditions on the ACnetwork which are non-ideal. For example, the preponderance ofuncontrolled diode-capacitor rectifiers in electronic products that arein general use today cause a well known depression of the crest of theutility AC voltage waveform, such rectifiers only drawing current at, ornear to, the crest of the utility waveform. This depression is reflectedto the utility as a requirement to supply principally third, some fifth,and other harmonic currents, raising the costs of generation andtransmission of electricity. The system which is the object of thisinvention can, however, be controlled to draw current of a preset formfrom the utility network. One such waveshape is shown in FIG. 10. Underthis regime, the energy drawn from the network by the system describedhere is shifted from the crest of the waveform, and indeed energy isdelivered back to the network for a brief period at the waveform crest.This strategy has the effect of supporting the network during the periodwhere uncontrolled rectifiers are drawing current, and also offers abrief depolarization period to the batteries under charge. The principaldrawback of such a scheme is the magnitude of the high frequencycurrents that are induced to flow within the batteries. The rectifiercan be programmed if required to draw other waveshapes with differentharmonic structures if desired, such programming also being possible byremote communications means if these are available.

Where the traction drive inverter is used as a regenerative rectifier inthe manner suggested by this invention, additional means are required tolimit source currents in the situation where the DC battery voltagelevel has fallen below the crest of the connected voltage source. Whileit is rare for the voltage of the traction batteries to fall below thesource voltage crest, such events are observed and can reasonably beexpected in vehicles which are (a) made available for use by the generalpublic, (b) equipped with high voltage traction batteries which havevoltages, in any case, with a normal value only slightly above thevoltage of typical incoming sources, (c) equipped with high temperaturebatteries or (d) heavily discharged. Where the vehicle is left idle andnot connected to a charging source for a period varying in length from afew days to perhaps several weeks, the charge level of the batterieswill become slowly depleted, either from supporting on-board systemsthat require continuous power, and/or from battery self dischargeeffects. In the case of high temperature batteries, cooling of thebattery will occur over this period. When such batteries reach thefrozen state, the voltage level falls essentially to zero. In most hightemperature battery management schemes, the battery energy is consumedto maintain battery temperature before the frozen state is reached, sothat the frozen state will be coincident with a low level of charge.Consequently, a regenerative rectifier charging system is not practicalfor use in normal vehicles without additional means for handling the lowbattery voltage condition, and indeed unless special steps are taken toaccount for this condition, large surge currents will result from theclosure of the switch 4 (see FIG. 7) to connect a vehicle in this stateto the normal charging source, with possible battery damage, andprotective systems such as fuses being called into play.

Such means could be implemented externally. For example, in thesituation where (say) the vehicles are forklifts or material handlingdevices used within a constrained area such as a factory, technicianscan bring special power converters and chargers to the dischargedvehicle for purposes of providing an initial boost charge. However, forvehicles used by the public, it is less convenient for intervention byservice technicians to be necessary, and the equipment should beprovided on the vehicle itself so that charging can be carried out bythe normal charging source, without any difference being visible to theuser.

These means can be implemented in various ways; either in series withthe regenerative rectifier on the source or battery sides, or inparallel with the main regenerative rectifier path. An additionalconverter (as described later) may be provided, in which case power forthe additional converter is drawn from the source side of the switch 4(FIG. 7). The series limiting means take the form of a variableimpedance which may be variously implemented using resistors, inductors,controlled semiconductor devices, or other methods.

For example, triac devices may be inserted in series with each incomingAC source line. The triac devices 17 in FIG. 11A are controlled by theinverter/rectifier 2 and microcontroller 3, under the well-knownphase-controlled regime, to reduce the effective AC input voltage--andtherefore control the battery currents--during the period until thebattery voltage rises above the crest voltage of the AC lines. Drawbacksof this system include the non-sinusoidal nature of the AC line currentsduring the period when the triacs are in operation. An alternative,illustrated in FIG. 11B, is the placement of a series element betweenthe inverter DC output terminals and the battery. The element 18 may beinserted in either the positive pole, as shown, or the negative pole, asrequired. The series device may conveniently be an arrangement of IGBTdevices, or similar semiconductor switches. The series device 18 ismodulated by the inverter microcontroller according to a pulse widthmodulated or other regime, to reduce the inverter DC output voltageuntil the battery voltage rises as before. A parallel device 19 isprovided for the control of currents flowing in inductances 20 on the DCside. A bypass switch 21 is used so that high rate charging is possibleduring normal operation of the system as a regenerative rectifier. Thismethod has considerable advantages over the triac method. In particular,AC line currents of high quality are still drawn during the initialcharging period, and the software required for the control of thisseries device is similar in many respects to the regenerative rectifiersoftware used for the bulk of the charging period.

In some circumstances, it is preferable to provide an additionalconverter, placed in parallel with the main regenerative rectifier, tocover the situation in which the voltage of both the auxiliary andtraction batteries is very low. The placement of the additionalconverter is illustrated by FIG. 12. This additional converter 24 isnormally controlled by the main inverter microcontroller under a similarregime as described above, but is capable of limited operation on itsown account. It is connected so as to charge both the main tractionbattery 1 and the auxiliary battery or batteries 12 (see FIG. 7 also).Such limited operation involves conversion of power at a low rate (say200 W). The additional converter contains sufficient circuitry toprovide this form of operation, which is invoked only by connecting itto a suitable charging source such as a single phase supply. Energy isthen converted from the charging source by the additional converteruntil the voltage of the auxiliary battery rises sufficiently thatoperation of the main inverter microcontroller becomes possible. Themain inverter microcontroller then modulates the additional converteraccording to a pulse width modulated or other regime, to improve thequality of the currents drawn from the charging source. Charging of themain traction battery is then possible via the additional converter athigher rates (say 1 kW) without undesirable levels of harmonic pollutionbeing imposed on the connected charging source. With typical tractionbatteries it is necessary for 2% or more of full charge to be suppliedbefore the open circuit voltage rises close to the normal levels.Consequently, at the 1 kW rate, it is necessary to operate theadditional converter for approximately 30 minutes before the tractionbattery voltage rises to the levels where normal charging, via theregenerative rectifier, can commence. Such operation is also compatiblewith the reheating requirements of high temperature batteries, such assodium sulphur or sodium nickel chloride batteries for example, whichmust be heated to the molten state before charging can commence.

In this way, it is possible to re-energize a vehicle which has becomecompletely "dead" (even to the extent that the main invertermicrocontroller has ceased operation), merely by connection of thenormal, or another suitable, charging source. Such operation occurscompletely without intervention from the vehicle user--although thelonger charging time will be apparent to the user--which is importantfor vehicles which are to be operated by the general public. Providedthe equipment described here is provided on the vehicle, it isunnecessary for any technician or service organization to be involved.The user can reenergize a deeply discharged vehicle in the normal way.

Drawbacks include the extra cost of the additional converter, althoughin practice this cost is minimised if the major control functions aredelegated to the microcontroller already used for controlling theinverter/regenerative rectifier.

The operation of the vehicle traction drive system shown in FIG. 14 ofthe drawings is generally as described above, but the system will now befurther described in relation to FIG. 13, specifically in relation tothe use of the vehicle charging system in reverse, to supply power fromthe vehicle battery to the utility grid or mains.

The particular arrangement of consumer units, meters, communicationfacilities will depend upon individual utility requirements, as will thecontrol software and communications link(s) to enable operation.

In particular, alternatives of a dedicated communications link or aripple signal decoder are illustrated. In other respects, the chargingsystem is as described above.

FIG. 13 is a simplified diagram of the grid (mains utility) system andassociated vehicle components. The diagram shows plural utilitygenerators 101, 102 normally supplying electrical power to a grid,generally indicated at 100, through conventional transformers 103 whichstep up the voltage as normal for transmission. Further transformers104, 105 step the voltage down to local and then domestic and industrialuser levels and the supply is fed to individual meters 106, 107 and thendistributed through conventional domestic or industrial consumer units108, 109. One dedicated outlet is an EV charging outlet 110 which can beconnected through a power cord 111 to an electric vehicle 112, thecomponents of which are generally as described above, but include acharging system 113 and traction battery 114.

A ripple signalling unit 115, located at the utility generator can beused to provide signals to corresponding vehicle units 14 to causeconnected vehicles to re-supply the grid as desired, but, alternativecommunications systems such as radio, cable, fiber-optic may beutilised. These links are indicated schematically at 117, 118 and thevehicle-end units for receiving the signals are indicated at 15 (seeFIG. 7).

In actual operation of the system to supply the grid or mains from agiven vehicle, in the vehicle the switch 4 (see FIG. 7) connects theconverter 2 to the utility supply through an impedance 6 as forcharging. The operation is substantially the same as for charging, inthat the converter-generated voltage is controlled to have a desiredvectorial relationship with the utility grid/network voltage. In otherwords, both real and reactive components which result when voltages andcurrents are not in phase may be controlled. In the case where power issupplied to the grid or mains, this relationship is different from thecharging case in that the converter-generated voltage is controlled sothat power is forced to flow from the battery through the converter intothe utility network. The vectorial relationship depends on therequirements for power level, power factor, and harmonic content. In oneimplementation of the present system, this requirement is transmitted,along either a dedicated communication link 15 or alternatively a ripplecommunication system 14, from the utility company or other authorisedcontrol authority.

In the context of the type of vehicle to which the present inventionprimarily relates, where a star connected three phase traction motor isbeing used, there is a straightforward method for providing a 12 Vsupply--or indeed any other appropriate voltage that may be desired.

The method used in this invention is to offset each motor pole voltageby a constant amount, as illustrated by FIG. 8. In a symmetricalmultiphase system, such offsets which are vectorially concurrent have nonett effect on the individual phases. Such concurrent disturbances causethe neutral point of a symetrical multiphase system to be displaced. Theeffect is commonly observed in multiphase, inverter-fed systems asvoltage oscillation of the neutral point, relative to the groundterminal of the DC supply, particularly where an asymetrical offsetoccurs per phase.

In the inverter of the present system, as illustrated by FIG. 7, anoffset is applied by the microcontroller 3 to the PWM sequencesgenerated for each inverter output pole. The offset is such that the theoutput PWM sequences contain a DC component which--when time-averaged bythe filtering effect of the motor windings and an additional filter11--equates to a neutral point offset voltage of nominally 14 V. Theneutral point of a star-connected motor is directly accessible and canbe used (with appropriate protection circuitry) for charging anauxiliary 12 V battery 12. Feedback via the traction drivemicrocontroller 3 is used to regulate the offset voltage, controllingthe auxiliary battery charging processes.

FIG. 8 illustrates the vectorial relationship of the pole voltages,leading to the neutral point offset.

Several operational points are important for the practical utilisationof this neutral point offset technique. In particular, a periodicreversal of the offset, with respect to the centerpoint of the tractionbattery, is required so that a balanced discharge of both halves of thetraction battery is achieved. Diodes 22 as shown in FIG. 15A can beadded to the filtering circuitry 11 of FIG. 7 so that unidirectionalcurrent flow through the auxiliary battery 12 is maintained in thesecircumstances. Furthermore, precise control of the pole to poleswitching periods of the pulse width modulated inverter is required,with the desired pulse period differentials required for the creation ofthe neutral point voltage offset being maintained through the powerstage to the motor terminals. In practice, the energy delivered to theauxiliary battery from the main traction battery via the inverter powerstage is in short pulses, so that the effects of source impedance areimportant. Furthermore, the traction motor must be rated to withstandthe additional winding losses that result from the extra currents thatflow as a consequence of this neutral point offset technique. Inpractice, this loss increase is of the order of 1% of the rated machinepower, depending on the level of low voltage power delivery, thespecific characteristics of the machine, and other factors, so that intypical circumstances no change to the machine design is required.

Where delta-connected motors are used, the neutral point is not directlyaccessible. An artificial neutral for charging purposes can beconveniently created by a number of star-connected reactive impedances.The impedances need only be rated to carry the anticipated auxiliarybattery charging currents.

In some systems, it is possible to modulate the pole offset at a highfrequency of several hundred Hertz, so that the charging source derivedfrom the neutral point offset can itself be passed through a small highfrequency transformer 23 (see FIG. 15B) to provide isolation ifrequired. A drawback of this secondary-isolated system in motor drivesintended for wide speed ranges is that the effective carrier frequencyavailable to support the neutral point oscillation becomes low at highmotor speeds, as the number and time duration of the PWM pulses used toform each cycle of the basal motor frequency becomes reduced. Theisolation tranformer must therefore be designed to support the lowestfrequency encountered over the full range of inverter operation, whichhas the effect of increasing the transformer size and weight. Thetransformer positioning is illustrated by FIG. 15B. In most systems,however, the effective frequency does not drop below 100 Hz, andproblems do not therefore arise.

Alternatively, the offset can be temporarily removed if the effectivefrequency falls below a set point. This means that charging theauxiliary battery will not occur during periods of high speed vehicleoperation, but that a high frequency isolation transformer is stillusable. This is unlikely to be a disadvantage for these vehicles, and ifnecessary (if charging is essential) the offset can be restored byintroducing additional PWM pulses, at the expense of underfluxing thetraction motor slightly.

We claim:
 1. In a battery powered electric vehicle, a system forcontrolling power transfer with a power distribution network forcharging and discharging the battery when connected theretocomprising:power transfer means for passing electric powerbi-directionally between the battery and the power distribution network;communication means operatively coupled between the power distributionnetwork and the power transfer means for providing control signalsindicative of at least one of a selected power factor, power level andharmonics content to the power transfer means in accordance with thesupply and demand on said power distribution network; and control meansresponsive to the control signals and operatively coupled to the powertransfer means for controlling the power passing between the battery andthe power distribution network.
 2. A system according to claim 1, whichis integral with an on-board vehicle electric traction drive system. 3.A system according to claim 1, wherein the battery charging/supplysystem includes a number of semiconductor switches.
 4. A systemaccording to claim 1 including a timing means for controlling powertransfer at selected times.
 5. A vehicle according to claim 1, furtherincluding a state-of-charge estimator to control the maximum charge anddischarge states of the battery.
 6. A system according to claim 1wherein the communication means comprises a channel separate from thepower distribution network.
 7. A system according to claim 1 wherein thecommunication means includes power lines of the power distributionnetwork for carrying the control signals superimposed thereon.
 8. Asystem according to claim 1 including an AC traction motor; a pulsewidth modulated (PWM) converter having first and second portscontrollable to convert a DC electrical signal, fed from the battery tothe first port, into an AC drive signal for the motor fed out from thesecond port; an AC input port; and switch means connected to the secondport, for switching the second port between the traction motor and ACinput port, whereby the AC input port, on connection to an AC source canbe connected to the converter to charge the battery.
 9. A systemaccording to claim 8, which further comprises plural DC link capacitorsfor sourcing and sinking high frequency current pulses that result fromthe operation of the power converter, the PWM converter including anunidirectional self-commutating semiconductor switches (Q₁ -Q₆) withanti-parallel diodes (D₁ -D₆), the switches being arranged in afull-wave bridge configuration.
 10. A drive system according to claim 9,including a microprocessor based controller for generating drive pulsesto provide gating signals to the switches (Q₁ -Q₆).
 11. A systemaccording to claim 8, including an electromechanical changeover switchcomprising a pair of mechanically interlocked contactors which allowconnection of the PWM converter to either the AC traction motor or theAC utility supply inlet port.
 12. A system according to claim 11,wherein the position of the changeover switch is controlled by themicroprocessor.
 13. A system according to claim 8, including aninterlock for inhibiting changeover occurring while the vehicle or thetraction motor is in motion.
 14. A system according to claim 8, whereinthe battery is of lead-acid, sodium sulphur, sodium nickel chloride orother type suitable for vehicle traction.
 15. A system according toclaim 8, including apparatus for providing an auxiliary DC voltagesupply for vehicle equipment, from the PWM controlled converter,including means for offsetting a neutral point of a multi-phase supplyby offsetting the PWM sequences for each inverter output pole.
 16. Asystem according to claim 8, including means for enabling the battery tobe charged through the PWM converter.
 17. A system according to claim16, wherein said means comprises a switching device positioned in seriesbetween at least one of the battery and the converter and between theconverter and the charging AC supply.
 18. A system according to claim16, wherein an auxiliary converter is provided, in parallel with themain converter, to enable charging of the battery independently of themain converter.
 19. A system for controlling power transfer between abattery powered electric vehicle and a power distribution networkcomprising:power transfer means for passing electric powerbi-directionally between the battery and the power distribution networkfor charging and discharging the battery; communication meansoperatively coupled between the power distribution network and the powertransfer means for providing control signals of a selected vectorialrelationship to the power transfer means in accordance with supply anddemand on said power distribution network said vectorial relationshipincluding at least one of a power level, power factor and harmoniccontent; and control means responsive to the control signals operativelycoupled to the power transfer means for controlling the power passingbetween the battery and the power distribution network in accordancewith said vectorial relationship.
 20. A system according to claim 7wherein the communication means includes at least one of a radio, cableand fiberoptic link.
 21. A system according to claim 20 wherein thecommunication means includes billing means.
 22. A system according toclaim 19 wherein the communication means includes a ripple generator.23. A system according to claim 1 wherein the power transfer meansincludes a three phase circuit.
 24. A system according to claim 1wherein a plurality of said battery powered vehicles, each having acharging system and batteries at various charging states are coupled tothe power distribution network and the control signals are selected foreach vehicle such that the plurality of charging systems cooperate toprovide load leveling of the power distribution network.
 25. A chargingsystem for controlling power transfer between a battery powered electricvehicle and a utility power grid subject to supply and demandcomprising:circuit means on the vehicle for passing electric powerbetween the battery and the power distribution network; communicationmeans operatively associated with the power grid for providing controlsignals to the circuit means in accordance with the supply and demand onsaid power distribution network; and control means responsive to thecontrol signals and operatively coupled to the power transfer means forcontrolling at least one of a power factor, power level and harmonicscontent of the power passing between the battery and the powerdistribution network.
 26. A system for charging a battery representing asensible load from energy supplied by a power grid and for dischargingthe battery to supply energy back to the grid comprising:circuit meansfor bi-directionally passing power between the battery and the grid inaccordance with supply and demand on said grid; control meansoperatively coupled to the circuit means for controlling charging anddischarging of the battery in accordance with control signalsselectively representing real and reactive components of the power to betransferred; and communication means between the power grid and thecontrol means for carrying said control signals to said control means.